In a stay-behind operation, a country places secret operatives or organisations in its own territory, for use in the event that an enemy occupies that territory. If this occurs, the operatives would then form the basis of a resistance movement or act as spies from behind enemy lines. Small-scale operations may cover discrete areas, but larger stay-behind operations envisage reacting to the conquest of entire countries.

In some cases, stay-behind operations have deviated from their stated purpose, and have become active against elements in their own countries which they deem to be subversive — rather than fighting an outright invasion, they claimed to be fighting a quieter subversion of their country.

In some countries, there has been a considerable degree of overlap between official stay-behind organisations and other, non-official groups. For example, the French opponents of Algerian independence in the Organisation de l'armée secrète included many members of its country's stay-behind organisation.[citation needed]

^Collins, John M. (1998). Military Geography. Potomac Books, Inc. p. 122. ISBN9781597973595. Retrieved 2013-09-28. The Pripet Swamp, which created a great gap between German Army Group Center and Army Group North soon after ... June 1941, made it impossible for large military formations to conduct mutually supporting operations. Attempts to bypass such extensive wetlands proved perilous, because outflanked Soviet stay-behind forces and partisans pounced on logistical troops as soon as German spearheads disappeared.